The present invention relates to a cooling system for a combined cycle gas turbine and particularly relates to integrated steam and air cooling for a gas turbine, a method of operating the system and various components of the system, including turbine blades adapted specifically for steam cooling and an arrangement of the nozzle and turbine stages for steam or air cooling.
The traditional approach for cooling gas turbine blades and nozzles is to extract air from a source at a sufficiently high pressure, e.g., by extracting air from the intermediate and last stages of the gas turbine compressor. A series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades; whereas, for nozzles, cooling air is supplied and controlled through external piping. The cooling air flow circuits bypass the combustors where heat is supplied to the thermodynamic cycle. Thus, the diverted coolant air does not receive energy directly from the combustors and does not completely expand through the turbine. This arrangement represents parasitic losses to the turbine output and degrades overall performance efficiency.
Steam cooling in reheat gas turbines has been previously discussed, e.g., see U.S. Pat. Nos. 4,314,442 and 4,565,490 to Rice. Steam cooling has also been discussed in a report by the Electric Power Research Institute, Report No. RP2620-1, entitled "Future Gas Turbine Development Options Definition Study," dated June 1987. This report describes the anticipated performance improvement for steam cooling from a thermodynamic cycle analysis perspective. In the context of that report, the steam cooling supply requirements included a very high pressure source, i.e., on the order of 1840 psia, because it was then believed that such high pressure was needed to overcome circuit friction losses, as well as adverse rotational and centrifugal field forces associated with that proposed closed cooling circuit configuration.
In a combined cycle operation, steam at several pressure and temperature levels is readily available. Coolant air in a gas turbine can be replaced by steam, which is the better cooling medium. Moreover, the problem of degradation of thermal efficiency associated with air as the cooling medium is ameliorated as the transition from air to steam cooling is performed. By using steam as coolant, it is also possible to increase the firing temperatures in the gas turbine cycle.
According to the present invention, it has been found desirable to integrate steam and air cooling in a combined cycle system where primary cooling is provided by steam and off-design operating conditions, e.g., start-up, is provided by air. That is, it has been found desirable to operate a gas turbine using steam cooling and to have available a backup for operational off-design cooling using air, for example, during start-up or an abrupt failure in the supply of steam. In accordance with this invention, an existing air-cooled gas turbine is modified to change over from operational air cooling to steam cooling. Thus, cooling flow distributions, particularly in the first and second-stage turbine blades and second-stage nozzles, require necessary modification to accommodate steam cooling.
More particularly, second-stage nozzle vanes and first-stage turbine blades are designed specifically to take advantage of the thermal efficiencies of steam cooling. In the second-stage nozzle, a pair of pipes or tubes extends from a manifold coupled to a suitable source of steam from the combined cycle operations and extend through the nozzle vanes and the diaphragm associated with the nozzle vanes. The inner surface of the diaphragm seals with the outer surface of a spacer in a conventional manner, the spacer being carried for rotation with and between the wheels mounting the first and second-stage turbine blades. The spacer defines a pair of chambers with the first and second-stage turbine wheels. Coolant steam passing through the nozzle vanes and through the diaphragm communicates with the chambers and with inlet ports for passage through the first and second-stage turbine blades, as described hereinafter.
Further, discrete inserts envelop and encompass each of the tubes through the nozzle vanes. Each insert is provided with a plurality of apertures for flowing air supplied to the space between the steam carrying tube and the insert outwardly into a cavity defined between the insert and the walls of the nozzle vane. The air cools the nozzle vane and exits the vane both through a series of apertures in its trailing edge and into a chamber within the diaphragm for exit in opposite axial directions into the gas flow through the turbine. The tubes conducting the steam have ribs about their external surfaces to improve the heat transfer relation between the steam within the tubes and the air flowing within the inserts. The external surfaces of the inserts are provided with ribbing, preferably spiral or helical, to direct the flow to the trailing edges and the diaphragm. In operation, the heat transfer between the steam and air lowers the temperature of the steam and increases the temperature of the air. The air flow, however, is expanded and cooled upon passing through the apertures in the insert for cooling impingement against the inside surfaces of the nozzle vanes.
The steam flows through the tubes and diaphragm and through the seal between the diaphragm and the spacer. Preferably, the seal is a labyrinth-type seal with multiple projecting teeth. In accordance with this invention, injector nozzles are spaced one from the other circumferentially about the sealing surface of the spacer. The steam flows from the diaphragm between the adjacent teeth of the labyrinth seal for flow through the injector nozzles in the spacer. The nozzles are shaped to accelerate the flow of steam into the chambers on opposite sides of the spacer.
Coolant steam for the first and second turbine stages is additionally inlet from a location adjacent the shaft of the turbine into the areas between the first and second-stage turbine wheels. Passages are provided through the spacer to enable the steam to enter the chambers. Thus, this inner steam flow passes radially outwardly by centrifugal force to mix with the steam input to the chambers from the tubes of the nozzle stage and the injector nozzles of the spacer. This combined steam flows through and cools the turbine blades of the first and second stages.
In a further aspect of the present invention, each first-stage turbine blade includes a serpentine cooling arrangement. In a preferred embodiment, this includes four coolant circuits: two single-pass radially outwardly directed passages adjacent the leading and trailing edges of the blade and two intermediate three-pass, forward and aft circuits. The inlet ports for the serpentine passages are through the pedestals mounting the turbine blades. With respect to the forward and aft intermediate circuits, the respective inlet ports are located in the root portion of the blade and the flow of steam is through passages first directed radially outwardly toward the tip portion, then radially inwardly toward the root portion and finally radially outwardly toward the tip portion to exit the turbine blade substantially medially of the blade at its tip portion. The steam therefore flows in serpentine fashion from adjacent the leading and trailing edges in opposite axial directions toward a mid-portion of the turbine blade. Thus, the steam which has collected the most heat from the blade advantageously exits the blade at a location which has the lowest metal temperature.
The leading edge circuit flows steam radially outwardly between an inlet port at the root portion of the blade and an outlet at the tip portion and through a plurality of radially spaced apertures opening into a recess on the leading edge of the blade. That recess is located along the stagnation or pitch area of the blade which is the area of highest blade temperature during operation. The recess contains a porous material, such as woven wire mesh of high density, whereby steam from the first leading edge circuit flows through the apertures into the recess through the mesh for transpiration cooling. The trailing edge circuit flows steam from an inlet port adjacent the root portion of the blade to an outlet adjacent the tip portion, as well as through a series of apertures radially spaced along the trailing edge of the blade.
Additionally, on the pressure side of each vane, there is provided a series of bleed film cooling holes radially spaced along the blade and in communication preferably with the first passageway of the forward intermediate circuit for supplying film cooling steam along the pressure surface of the vane. Film cooling is provided because steam has superior radiant properties, e.g., absorbtivity and emissivity, and absorbs much of the radiant energy and emits this energy at a lower intensity. On the pressure side of the vane, there is also provided a series of bleed film cooling holes radially spaced along the blade, preferably in communication with the final passageway of the aft intermediate circuit. The location of these bleed film cooling holes between the leading and trailing edges of the blade on the suction side is selected because the boundary layer thickens along this area. The boundary layer increases the convective thermal load on the part. By reducing the boundary layer by thin film cooling, the convective thermal load on the part is reduced.
The second-stage turbine blades are each provided with a plurality of straight-through radial passages for passing cooling steam radially outwardly to the blade tips. Each turbine stage has a steam collection shroud adjacent the tips of the blades for collecting the cooling steam.
In accordance with this invention, an air cooling system is integrated with the steam cooling system just described. To accomplish this, a rotating nozzle collar is provided on the inner circumference of the first-stage wheel. Fixed and movable valve structures are mounted about the shaft. The valve is normally closed to prevent air under pressure from the compressor from flowing radially outwardly into the spaces between the wheels and spacer and into the chambers. During start-up or off-design operation, for example, when steam pressure is not available or lost, the solenoid is actuated to open the valve to provide air under pressure into those areas for flow through the inlet ports of the first and second-stage turbine blades to effect cooling. After start-up or when the air cooling is generally not needed, the solenoid closes the valve to prevent air from entering those spaces.
In a preferred embodiment according to the present invention, there is provided a cooling system for a gas turbine comprising a pair of axially spaced, rotatable turbine stages, each having a plurality of turbine blades for disposition in a gas flow through the turbine, the turbine blades each having at least one interior passage extending from an inlet port adjacent a root portion of the blade to an outlet port adjacent a tip portion of the blade. A nozzle stage is provided between the turbine stages and includes a plurality of nozzle vanes for disposition in the gas flow, with each vane having at least one interior passage having an inlet and an outlet adjacent respective radially outer and inner end portions of the vane for passing steam from the inlet radially inwardly through the vane to the outlet. Also provided is a spacer between the turbine stages and rotatable therewith, with at least one passageway through the spacer and in communication with the outlets for the vanes and the inlet ports for the turbine blades for flowing steam from the nozzle vanes to the turbine blades, whereby steam may flow through the passages in the blades to cool the blades.
In a further preferred embodiment according to the present invention, there is provided a blade for the turbine stage of a gas turbine, comprising a turbine blade body having a discrete length and a general airfoil shape, with pressure and suction sides, tip and root portions and leading and trailing edges, a plurality of discrete internal passages extending lengthwise along the blade having inlet and outlet ports adjacent the root and tip portions, respectively, for flowing cooling fluid through the blade including a pair of single-pass passages adjacent the leading and trailing edges, respectively, and at least one multi-pass passage intermediate the leading and trailing edges and the single-pass passages. The multi-pass passage has at least three discrete passageways for flowing cooling fluid from the root portion toward the tip portion, back toward the root portion and again toward the tip portion to cool intermediate portions of the blade.
In a further preferred embodiment according to the present invention, there is provided a method for cooling a multi-stage gas turbine having a pair of rotatable turbine stages, each having a plurality of turbine blades, a nozzle stage between the turbine stages and having a plurality of nozzle vanes, and a spacer between the turbine stages inwardly of the nozzle stage, comprising the steps of flowing steam radially inwardly through the nozzle vanes and through apertures in the spacer into a pair of chambers on opposite sides of the spacer and flowing steam from the chambers into cooling passage within the turbine blades to cool the blades with the steam exiting the blades adjacent the tips of the blades.
Accordingly, it is a primary object of the present invention to provide a novel and improved steam cooling system for combined cycle gas turbines with integrated air cooling for start-up or off-design operating conditions and methods of operation.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.